“That kind of behavior would be totally unacceptable where I come from.”

You went to a supermarket in the U.S. and the cashier looked bored and angry. You say this because you can't believe that a store employee would be that rude.

That kind of behavior would be totally unacceptable where I come from.

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that kind of behavior

A person's "behavior" is the way that they act. You can have "good behavior" or "bad behavior". But the phrase "that kind of behavior" always refers to someone who's doing bad things:

I do not engage in that kind of behavior. Unfortunately, that kind of behavior is common in large companies.

(something) would (be something / do something)

Say that something "would be ___" to describe an imaginary situation.

For example:

If I were rich, I would travel around the world non-stop.

In this example, the speaker is imagining having a lot of money, which isn't really the case.

Here's another example:

Don't you think that you would get tired of talking with fans if you were a celebrity?

You can also use "would" to talk about something that might happen in the future, but isn't certain yet.

If you sold your business, what would you do for work?

If you went to Spain, where would you go?

(something) is totally unacceptable

Use this phrase to talk about things that someone must not or should not do. Parents, teachers, managers, and other people with authority use "totally unacceptable" when they're yelling at someone that they're in charge of. Customers also use it to complain about problems they have with a business. For example, if your luggage gets lost by an airline company, you might tell an airline employee:

This is totally unacceptable!

This phrase sounds formal, but it's also a very strong criticism, so only use it when someone has done something really wrong and you're angry with them.

where I come from

You can use the phrase "where I come from" to talk about differences between your home town, region, or country and the place where you are now:

Where I come from, people say "Excuse me" when they bump into you.

People usually use this phrase when they're complaining about things that people do, which are considered rude in the place that they're from. It's common for people from small towns to say things like this when they go to a big city.